1. Field of the Invention
The present invention is directed generally to semiconductors and more specifically to a tunable semiconductor laser diode.
2. Description of the Related Art
Laser light sources that are tunable over a broad range are required for advanced optical communication systems. Simple operation and a broad tuning range are required of such laser diodes. A tunable laser diode that is divided into an amplifier region, a coupler region and an absorber region over its length and that is referred to as an ACA laser is described in the publication by S. Illek et al., "Co-Directionally Coupled Twin-Guide Laser Diode for Broadband Electronic Wavelength Tuning" in Electronics Letters 27, 2207-2208 (1991). Two waveguides are arranged vertically relative to one another and separated from one another by a cladding layer. Light that propagates in a first mode is generated and amplified in the upper waveguide. This wave is coupled into the lower waveguide in the coupler region. The absorber region prevents the emission of the mode in the upper waveguide. Laser mode becomes possible due to the exchange of the radiation capacity between the two modes guided in the two waveguides.
The publication by I. Kim et al., "Broadly Tunable InGaAsP/InP Vertical-Coupler Filtered Laser With Low Tuning Current" in Electronics Letters 29, 664-666 (1993) discloses a VCF laser diode wherein an amplifier region, a coupler region and what is referred to as a window region are arranged following one another. The amplifier region and the coupler region have separate power terminals. The amplifier region has a quantum well layer structure. The suppression of longitudinal side modes is relatively slight given the two structures disclosed in said publications since the wavelength-selective filtering and tuning occurs only in a sub-area of the overall laser diode. This can lead to an inadequate and unstable single-mode emission.
This problem can be essentially avoided when the filtering is distributed over the entire laser length, as, for example, in the DFC laser structure from the publication by M. C. Areann et al., "Widely Tunable Distributed Forward Coupled (DFC) Laser" in Electronics Letters 29, 793-794 (1993). Two waveguides are likewise arranged vertically relative to one another and coupled in this structure. Absorption regions that form a filtering grating in longitudinal direction are arranged vertically relative thereto in longitudinal direction.
Another proposed solution for suppressing side modes is disclosed in the publication by M. Oeberg et al., "74 nm Wavelength Tuning Range of an InGaAsP/InP Vertical Grating Assisted Codirectional Coupler Laser with Rear Sampled Grating Reflector" in IEEE Photonics Technology Letters 5, 735-738 (1993). The functions of filtering and tuning are thereby separated from one another and are separately optimized. As in the lasers set forth above, the codirectional mode coupling with its relatively slight spectral filtering of the (spectrally proximate) side modes is employed for broadband tuning. The far selection for modes at a greater spectral distance is good in the codirectional mode coupling (particularly when the coupler length corresponds to a coupling length). A great suppression of the side modes is therefore achieved by the additional employment Of a specific Bragg grating structure as laser end mirror. The good near selection of this specific Bragg grating, however, is acquired at the expense of a poor far selection (ambiguity). Due to the good far selection of the codirectional coupler, however, this does not mean a deterioration of the operating properties will result for the overall laser structure. The critical disadvantage of this combination--in addition to the relatively great length--is the critical synchronization of the filter function with the tuning function. This requires an extremely exact adjustment of two control currents and has a decisively negative effect on practical manipulation.